As organopolysiloxane oils possess good heat-resistant and release properties, they find wide application in toner release agents for Xerography, as well as in mold-release agents used in molding thermoplastic, thermosetting resins, or various rubbers. However, when dimethylpolysiloxane oils, which are typical representatives of organopolysiloxane oils, are used in mold-release agents, they are often subject to thermal decomposition because the temperature on the walls of the mold often exceeds 150.degree. C. When this temperature exceeds 200.degree. C., the thermal decomposition process is vigorously intensified, and this causes problems associated either with the formation of a viscous gel-like composition, or with the formation of a formaldehyde that possesses harmful properties. Furthermore, in high-speed Xerography (hereinafter referred to as "electrophotography") heat rollers operate under high-temperature conditions of about 200.degree. C. Therefore, when dimethylpolysiloxane oils are used as toner-release agents, they are subject to thermal decomposition which is accompanied by the formation of formaldehyde. The recent demand for an increase in mold production and for a decrease in fixation, led to an increase in the temperature of molds and heat rollers of electrophotographic equipement. In response to this demand, the authors have developed a release agent (see Japanese Laid-Open Patent Application (Kokai) 7-216376) with improved heat-resistant properties, this agent comprising a diorganopolysiloxane which contains phenol groups.
However, the aforementioned release agent often contains dimethylsiloxane oligomers with less than 20 silicon atoms in an amount of 20,000 to 200,000 ppm. Therefore, when this agent is used at high temperatures over a long period of time, the aforementioned dimethylsiloxane oligomers are evaporated and either contaminate surfaces, e.g., of exhaust fans, with a sticky substance, or the oily substance pours down and contaminates the mold. Another serious problem is that the aforementioned dimethylsiloxane oligomer is oxidized and turns into a silicon-dioxide powder which is harmful to electronic components that are in the vicinity. In particular, it is difficult to obtain clear copies when the silicon dioxide powder adheres to the surface of a corotron in electrophotographic equipment.